疏松金属材料冲击温度理论分析

以热力学原理和固态物质的三项式物态方程为基础,由密实物质的冲击绝热线和热力学状态,通 过等容线法推导出了疏松金属材料的冲击温度理论计算方法。以铁为例,分析了几种物理参数对该模型计算 结果的影响。计算和分析结果显示,利用新模型得到的计算结果与已有实验结果吻合较好,误差均在5%以 内。疏松金属材料的冲击温度受Grneisen系数、电子Grneisen系数影响不大,而密实度、冲击压力和电子 比热系数则会对疏松金属材料冲击温度产生较大影响。     

The isotope effect of gaseous hydrogen under shock compression

The shock compression method has been used to measure the Hugoniot data and shock temperature for gaseous hydrogen samples, covering the pressure range of 55-140 MPa and the temperature range of 3400-4500 K and with the initial conditions of P ₀ = 0.6 MPa, 1.2 MPa and T ₀ at room temperature. Spectral radiance histories emitted from shocked D ₂ and H ₂ + D ₂ (equimolar mixture) are monitored by a pyrometer system with seven wavelength channels. Theoretical calculations based on the Saha model with Debye-Hückel correction for the shock compression behavior of shocked gaseous samples are in good agreement with the measured Hugoniot data, but show slightly higher values for the shock temperature when comparing with experiments. An isotope effect relevant to these shocked hydrogen species has been found in the linear shock velocity vs particle velocity relation, in which the correlation factor between these hydrogen isotopes or hydrogen mixtures is simply of initial density dependence.Received: 8 December 2002, Accepted: 8 May 2003, Published online: 2 September 2003PACS: 62.50 + p, 31.30.GS, 51.90. + r     

On the structure of the Hugoniot relation for a shock-induced martensitic phase transition

The Hugoniot curve relates the pressure and volume behind a shock wave, with the temperature having been eliminated. This paper studies the Hugoniot curve behind a propagating sharp interface between two material phases for a solid in which an impact-induced phase transition has taken place. For a solid capable of existing in only one phase, compressive impact produces a shock wave moving into material, say, at rest in an unstressed state at the ambient temperature. If the specimen can exist in either of two material phases, sufficiently severe impact may produce a disturbance with a two-wave structure: a shock wave in the low-pressure phase of the material, followed by a phase boundary separating the low- and high-pressure phases. We use a theory of phase transitions in thermoelastic materials to construct the Hugoniot curve behind the phase boundary in this two-wave circumstance. The kinetic relation controlling the evolution of the phase transition is an essential ingredient in this process.      

Shock Hugoniot for Biological Materials

This paper presents shock Hugoniot compression data for several bio-related materials by using flat plate impact experiments. Shock pressure covered in this study ranges at least up to 1 GPa. It is emphasized here that spatial and temporal uniformity of pressure distribution behind a shock wave front is very important and it can be realized in these materials by the impact method, and their precise shock propagation characteristics have been obtained by the application of the procedure developed previously by our group. Hugoniot measurements for different and systematic data for various samples are compared with shock Hugoniot curve for water. Samples used in the experiment include water gel of gelatin, NaCl aqueous solution, and finally chicken breast meat. Several samples with different initial density were used for gelatin and NaCl solution. Shock Hugoniot function for all the specimens tested can be summarized as u _s =A+B u _p , B ~ 2Value of the intercept of the relationship, A, which has the meaning of the sound velocity, is apparently dependent on the material and ambient temperature. Physical meanings of the obtained results have been discussed.     

Dynamic crushing of aluminum foams: Part I – Experiments

The two-part series of papers presents the results of a study of the crushing behavior of open-cell Al foams under impact. In Part I, direct and stationary impact tests are performed on cylindrical foam specimens at impacts speeds in the range of 20–160 m/s using a gas gun. The stress at one end is recorded using a pressure bar, while the deformation of the entire foam specimen is monitored with high-speed photography. Specimens impacted at velocities of 60 m/s and above developed nearly planar shocks that propagated at well-defined velocities crushing the specimen. The shock speed vs. impact speed, and the strain behind the shock vs. impact speed representations of the Hugoniot were both extracted directly from the high-speed images. The former follows a linear relationship and the latter asymptotically approaches a strain of about 90% at higher velocities. The Hugoniot enables calculation of all problem variables without resorting to an assumed constitutive model. The compaction energy dissipation across the shock is shown to increase with impact velocity and to be significantly greater than the corresponding quasi-static value. Specimens impacted at velocities lower than 40 m/s exhibited response and deformation patterns that are very similar to those observed under quasi-static crushing. Apparently, in this impact speed regime inertia increases the energy absorption capacity very modestly.     

水中金属丝电爆炸动力学过程的零维模型

金属丝电爆炸包含丰富的物理内容,近年来国内的实验和理论研究取得了重要进展,理解该过程有助于完善Z箍缩及磁加载等离子体动力学过程的物理建模,校验物性参数。在自相似运动假设条件下,发展了冷启动计算的水中电爆炸丝零维动力学模型。从一维磁流体模型出发,推导了描述丝等离子体膨胀的零维动能方程和内能方程,采用实际气体状态方程和修正的李-莫尔电导率参数作为封闭条件,根据质量守恒及水中激波雨贡纽关系式获得了丝等离子体的边界条件。应用于水中铜丝电爆炸动力学过程和能量转化分析,结果表明：该零维模型的物理假设合理,在一定范围内改变丝直径等参数可产生不同的放电模式,与一维模型及实验结果符合较好,能够为同类实验的优化设计和数据分析提供参考。     

Spontaneous acoustic emission from strong shocks in solids

Summary  The role of free electrons in the stability of strong shock waves in metals under spontaneous acoustic emission is investigated. For that purpose, a three-term form of the equation of state is employed in order to describe the cold pressure, the thermal atomic pressure and the thermal pressure of free electrons. The equation of state enables the calculation of the sound velocity behind the shock, which in turn is utilized in the Dyakov–Kontorovich criteria for the shock stability. The integral over the Fermi–Dirac distribution function that describes the specific internal energy of free electrons is replaced by a model algebraic function that possesses correct asymptotic limits at low and high temperatures. It is shown that strong shock waves in all metals are prone to instability under spontaneous emission. However, the threshold for that instability is shifted to higher Mach numbers if free electrons are taken into account. It is further shown that the stabilizing effect of free electrons is vastly overestimated if the expressions for degenerate electron gas are employed for temperatures that are larger than the Fermi temperature. Received 22 November 1999; accepted for publication 12 July 2000     

Chemical relaxation in a viscous shock

We consider the flow of a nonequilibrium dissociating diatomic gas in a normal compression shock with account for viscosity and heat conductivity. The distribution of gas parameters in the flow is found by numerically solving the Navier-Stokes and chemical kinetics equations. The greatest difficulty in numerical integration comes from the singular points of this system at which the initial conditions are given. These points lead to instability of the numerical results when the problem is solved by standard numerical methods. An integration method is proposed that yields stable numerical results-continuous profiles of the distribution of the basic gas parameters in the shock are obtained.We consider steady one-dimensional flow in which the gas passes from equilibrium state 1 to another equilibrium state 2, which has higher values for temperature, density, and pressure. Such a flow is termed a normal compression shock.The parameter distribution in normal shock for nonequilibrium chemical processes has usually been calculated [1–3] without account for the transport phenomena (viscosity, heat conduction, and diffusion). The presence of an infinitely thin shock front perpendicular to the flow velocity direction was postulated. It was assumed that the flow is undisturbed ahead of the shock front. The gas parameters (velocity, density, and temperature) change discontinuously across the shock front, but the gas composition does not change. The composition change due to reactions takes place behind the shock front. The gas parameter distribution behind the front was calculated by solving the system of gasdynamic and chemical kinetics equations using the initial values determined from the Hugoniot conditions at the front to state 2 far downstream.Several studies (for example, [4, 5]) do account for transport phenomena in calculating parameter distribution in a compression shock, but not for nonequilibrium chemical reactions. These problems are solved by integrating the Navier-Stokes equations continuously from state 1 in the oncoming flow to state 2 downstream.We present a solution to the problem of normal compression shock in nonequilibrium dissociating oxygen with account for viscosity and heat conduction using the Navier-Stokes equations.     

多孔材料冲击特性的数值研究

使用物质点方法对多孔材料的冲击响应过程进行模拟研究.重点考察了孔隙度、冲击波强度和空隙平均大小对波后物质状态参量的影响,计算了局域旋度和散度随时间的演化及其涨落. 研究表明,在孔隙度较低时, 波后系统可以达到一个近似动力学平稳态,下游区域的空隙反射回的稀疏波使得物理量随时间轻微振荡.在孔隙度较大时, 波后物质不易被压实, 冲击波的衰减明显.在孔隙度一定的前提下, 更大的平均空隙导致波后系统温度的进一步提高.冲击波的衰减速率依赖于孔隙度、平均空隙大小和冲击波强度.局部湍流和体积耗散是非均匀材料中机械能转化为热能的重要机制;加载的冲击波越强, 材料的多孔特征越显著.压强、密度、温度、粒子速度趋于或远离平衡的节奏并不同步.      

Fracture in metal powder compaction

This paper presents a preliminary assessment and qualitative analysis on fracture criterion and crack growth in metal powder compact during the cold compaction process. Based on the fracture criterion of granular materials in compression, a displacement based finite element model has been developed to analyse fracture initiation and crack growth in metal powder compact. Approximate estimation of fracture toughness variation with relative density is established in order to provide the fracture parameter as compaction proceed. A single crack initiated from the boundary of a multi-level component made of iron powder is considered in this work. The finite element simulation of the crack propagation indicates that shear crack grows during the compaction process and propagates in the direction of higher shear stress and higher relative density. This also implies that the crack grows in the direction where the compaction pressure is much higher, which is in line with the conclusion made by previous researchers on shear crack growth in materials under compression. In agreement with reported work by previous researchers, high stress concentration and high density gradient at the inner corner in multi-level component results in fracture of the component during preparation.     

A physical model of the structure and attenuation of shock waves in metals

In this paper, a physical model of the structure and attenuation of shock waves in metals is presented. In order to establish the constitutive equations of materials under high velocity deformation and to study the structure of transition zone of shock wave, two independent approaches are involved. Firstly, the specific internal energy is decomposed into the elastic compression energy and elastic deformation energy, and the later is represented by an expansion to third-order terms in elastic strain and entropy, including the coupling effect of heat and mechanical energy. Secondly, a plastic relaxation function describing the behaviour of plastic flow under high temperature and high pressure is suggested from the viewpoint of dislocation dynamics. In addition, a group of ordinary differential equations has been built to determine the thermo-mechanical state variables in the transition zone of a steady shock wave and the thickness of the high pressure shock wave, and an analytical solution of the equations can be found provided that the entropy change across the shock is assumed to be negligible and Hugoniot compression modulus is used instead of the isentropic compression modulus. A quite approximate method for solving the attenuation of shock wave front has been proposed for the flat-plate symmetric impact problem.     

纯钒在冲击加载下的动态拉伸断裂和弹性波衰减特性

利用平板撞击和激光干涉测速技术,实验研究了国产热等静压纯钒在压力5.2~9.0 GPa、拉伸应变率0.47×105~1.19×105 s-1冲击加载下的层裂特性。结果表明:国产热等静压纯钒具有较强的抗动态拉伸断裂能力,其层裂强度在4.0~5.3 GPa范围,明显高于相似加载条件下文献给出的熔炼钒结果,这主要与热等静压加工工艺下纯钒杂质含量更低、内缺陷更少有关;同时,纯钒层裂强度对冲击压力和拉伸应变率均比较敏感。此外,对弹塑性加载速度剖面的分析发现:在6 mm样品厚度范围,纯钒的弹性波幅值随样品厚度增大而减小,雨贡纽弹性极限随样品厚度的衰减规律较好地满足指数关系σ_HEL=3.246（h_s/h₀）-0.386,h₀为单位长度。     

Generalized shock hugoniot of condensed substances

It is shown that in order to predict the shock Hugoniot of any substance up to a compression ratio equal to two it is sufficient to know the initial density and the initial compressibility. The possibility of finding a priori the equations of state of nonporous mixtures of two substances, porous samples, and solutions is discussed.     

Effects of multiple cracking on the residual strength behavior of thermally shocked functionally graded ceramics

When subjected to severe thermal shocks a functionally graded ceramic (FGC) suffers strength degradation due to the thermally-induced damages in the material. Multiple surface cracking has been observed as one of the dominant defects/damages affecting the thermal shock behavior of ceramics. This paper presents a thermo-fracture mechanics model to investigate the thermal shock residual strength behavior of elastically homogeneous but thermally graded FGCs undergoing multiple surface cracking. We consider an FGC plate with an array of parallel edge cracks at the thermally shocked surface. A Fourier transform/superposition method is used to derive the singular integral equation of the thermal shock crack problem. The critical thermal shock that causes crack propagation and thermal shock damage are determined using linear elastic fracture mechanics. The thermal shock residual strength of the FGC as a function of thermal shock severity and crack density (crack spacing) is subsequently evaluated. Numerical calculations are carried out for two FGC materials, i.e., Al₂O₃/Si₃N₄ and TiC/SiC FGCs, to illustrate the effects of crack density (crack spacing) and material gradation on the thermal shock strength behavior of FGCs. It is found that a higher crack density (lower crack spacing) together with appropriately graded material properties significantly enhances the residual strength of the thermally shocked FGCs.     

冲击温度的近似计算方法

将冲击温度的计算归纳为三种近似方法，并对这三种方法进行了概述，同时还给出了一些材料参数的估算方法．在利用等熵线计算冲击温度时，从冲击绝热线出发推导了一个半解析的等熵方程．计算了铁的冲击温度，并与实验测量值作了比较．结果表明，利用三项式物态方程并考虑熔化相变潜能的影响后算得的冲击温度与测量值符合得比较好，另外，本文还对影响冲击温度计算值的若干因素进行了分析．     

Thermodynamic conditions of shock Hugoniot curves in hot dense matter

We use a self-consistent model combining the Quotidian Equation Of State and a nonrelativistic screened-hydrogenic model with ?-splitting to study the thermodynamic conditions encountered along shock Hugoniot curves. We focus on the maximum compression ratio reached on shock Hugoniot curves for simple elements that are solid at normal temperature and pressure. Electron shell effect and pressure ionization can have a strong impact on the results. Numerical calculations are presented and discussed. Some of the physical points addressed in this work could be tested experimentally when the National Ignition Facility laser in the USA attains full power or when the Laser Mégajoule in France comes online.     

Shock waves in molecular solids: ultrafast vibrational spectroscopy of the first nanosecond

A novel technique which uses a microfabricated shock target array assembly is described, where the passage of a shock front through a thin (0.5μm) polycrystalline layer and the subsequent unloading process is monitored in real time with ultrafast coherent Raman spectroscopy. Using a high repetition rate laser shock generation technique, high resolution, coherent Raman spectra are obtained in shocked anthracene and in a high explosive material, NTO, with time resolution of ∼ 50 ps. Spectroscopic measurements are presented which yield the shock pressure (up to 5 GPa), the shock velocity (∼ 4 km/s), the shock front risetime (t _r < 25 ps), and the temperature (∼ 400°C). A brief discussion is presented, how this new technique can be used to determine the Hugoniot, the equation of state, the entropy increase across the shock front, and monitor shock induced chemical reactions in real time. Received 28 October 1996 / Accepted 12 November 1996     

激光冲击下CoCrFeMnNi高熵合金微观塑性变形的分子动力学模拟

激光冲击强化技术可以有效地提高材料的疲劳寿命, 被广泛应用于航空航天领域. CoCrFeMnNi高熵合金作为一种经典的高熵合金体系, 研究其激光冲击强化后的微观组织变化以及冲击动态响应对该材料未来在航空航天领域中的应用具有重要意义. 采用分子动力学方法, 对CoCrFeMnNi高熵合金进行了冲击模拟, 发现冲击时弹、塑性双波分离现象以及微结构演化具有明显的取向相关性. 沿[100]方向进行冲击时未出现双波分离结构, 并且塑性变形过程中会产生中间相; 而沿[110]与[111]方向冲击时产生了双波分离结构, 并且受冲击区域存在大量的层错以及无序结构, 高位错密度是产生无序结构的重要原因. 双波分离现象与可开动滑移系个数有关, 而沿不同取向冲击时的Hugoniot弹性极限和发生塑性变形的临界冲击速度与其可开动滑移系的Schmid因子大小有关. 此外, 冲击诱导了梯度位错结构的产生, 位错密度沿冲击深度先增加后减小, 在沿原子密排方向冲击时产生了更高的位错密度. 冲击之后在模型两侧存在残余压应力, 芯部为残余拉应力, 残余应力的大小具有明显的取向相关性. 最后, 与具有相同尺寸及取向的纯Ni进行对比, 发现CoCrFeMnNi高熵合金在冲击过程中由于晶格畸变效应产生了较纯Ni更多的无序结构.      

Shock compression of some porous media in conical targets: numerical study

Axially symmetric flows in conical solid targets filled by porous aluminum, graphite or polytetrafluoroethylene under impact of an aluminum plate with the velocity of 2.5 km/s are simulated numerically within the framework of the model of the hypoelastic ideal-plastic solid. The porosity of the samples is taken into account by conservation laws at the leading shock wave; the medium behind that is supposed to be nonporous. Equations of state for all materials in question are used to describe thermodynamic properties of the impactor and target over a wide range of pressures and temperatures taking into account phase transformations. The maximal over space and time pressure as a function of the initial relative density is presented and discussed.